This invention relates to the generation of small, widely spaced droplets of uniform size and more particularly, to the generation of such droplets by placing the end of a capillary tube in a venturi throat.
A need exists in laboratory experimentation and in scientific analyses for the production of a stream of uniform, isolated droplets of liquid. Streams of small, uniformly-sized droplets are required, for example, in testing aerosol standards for environmental toxicological studies, ink jet printing methods, new nebulization techniques for flame atomic absorption spectroscopy, novel means for investigating fast chemical reaction kinetics, and new approaches to studying the combustion behavior of fuels. Several techniques are currently used for producing streams of small uniform droplets suitable for such applications. See, for example, J. J. Sangiovanni and A. S. Kestin, Combustion Science and Technology 16, 59 (1977); J. C. Lasheras, A. C. Fernandez-Pello, and F. L. Dryer, Combustion Science and Technology 21, 1 (1979); C. H. Wang, X. Q. Liu and C. K. Law, Paper No. 82-81, Fall Western States Meeting, The Combustion Institute, Sandia Laboratories, Livermore, CA Oct. 11-12, 1982.
The prior art includes Rayleigh instability and mechanical chopping techniques of producing droplets which rely on physically breaking up an established high velocity liquid jet by electromagnetically induced mechanical vibrations or high speed rotary shuttering, respectively. Piezoelectric techniques can be used to expel droplets by mechanical constrictions applied to a liquid reservoir. The use of moving parts or electronic components for the production of droplets gives rise to the possibility of component failure, or drift, which adversely affects repeatability with time.
A major restriction to the use of Rayleigh instability for exploring isolated droplet combustion is the limited droplet spacing inherently available. However, no alternative droplet generation techniques (e.g., mechanical chopping or piezoelectric techniques) producing well-spaced (.gtoreq.30 droplet diameters) droplets have been described which are adaptable for use in combustion apparatus without introducing significant experimental complexities. J. A. Bolt and M. A. Saad, "Combustion Rates of Freely Falling Fuel Droplets in a Hot Atmosphere", Sixth Symposium (International) on Combustion, p. 717, The Combustion Institute, 1957, describes an apparatus for forming fuel droplets by a small bore tube from which the drops are thrown by a concentric jet of air. Droplet size is controlled by capillary bore size, pressure of the concentric air head, head of the fuel and the distance the capillary protrudes beyond the air nozzle. Resultant droplets are.about.1.1 mm in diameter, much larger than those found in typical combustion systems. In the laboratory, as well as practical combustion devices, there exists a limited droplet residence time within the combustor during which complete burnout must be achieved. Very large droplets (such as those described above) would require inordinately large combustion chambers to approach complete or nearly complete burnout. Note that the device of Bolt and Saad resulted in droplets which burned to.about.300 .mu. diameter over the length of their combustor, far from complete burnout.
It is an object of the invention to provide apparatus for producing uniform droplets of controlled, substantially smaller size in which the droplet spacing, and initial velocity can be regulated.